This invention relates to optical means for detecting the presence or position of an object and, in particular, to optical means for detecting the presence or position of a semiconductor wafer.
In the prior art, a variety of schemes have been proposed or even tried for determining the placement of an object, e.g. a semiconductor wafer. One such scheme is purely mechanical, i.e. the wafer is positioned by means which force the wafer to a certain location; for example, tines or a conical depression. Since such handling will likely result in the generation of small particles, more gentle handling and optical detection are usually used.
As used herein, unless otherwise indicated, "light" refers to that part of the electromagnetic spectrum to which semiconductive material is or can be made photosensitive. As such, it includes at least the visible and infra-red portions of the spectrum.
One problem with optical detectors is that the wafer, e.g. silicon, is transparent at infra-red wavelengths commonly used for emitters. Another problem is that scattering of the light by the surface of the wafer causes erroneous readings in the detector circuitry. Similarly, destructive interference of the light by various layers on the wafer can cause erroneous readings. Further, the alignment of the emitter and detector is often a problem due to vibration or use of the equipment incorporating the detector.
In view of the foregoing, it is therefore an object of the present invention to provide an improved optical presence or position detector.
Another object of the present invention is to provide an improved optical detector for use with semiconductor wafers.
A further object of the present invention is to provide an improved optical detector for use with surfaces having one or more layers thereon which can cause destructive interference.
Another object of the present invention is to provide an improved optical detector having better alignment control.